The accuracy of treatment planning system's (TPS) external photon beam dose calculations for complex fields involving the use of blocks, multi-leaf collimator (MLC), virtual wedges and asymmetric jaws are evaluated in this study. The treatment planning systems investigated were: (1) Helax-TMS (Version 4.1) which implements an energy fluence based model of the treatment head of a linac, and pencil beam algorithm for determination of the dose in the patient; and (2) CMS FOCUS (Version 2.5) which employs the Clarkson's algorithm and a scatter phantom ratio model to calculate the dose. Measured output factors (OF) and head scatter factors (HSF) for several square fields, fields formed by independent jaws, MLC formed fields and blocked fields were determined and compared with OFs and HSFs predicted by the TPSs. OR for virtual wedges, formed by independent jaws, were also measured and compared with those predicted by CMS and TMS. In addition, a method of independent verification of the monitor unit calculations of TMS was developed. (Abstract shortened by UMI.)